1. Field of the Invention
The present invention relates to an image forming apparatus in which light beams from a light source are radiated onto a photosensitive drum through a rotating polygon mirror to effect an optical scanning, and more particularly, so as to form an electrostatic latent image on the photosensitive drum.
2. Description of the Prior Art
One example of an image forming apparatus employing an optical scanning is a laser beam printer. A laser beam printer uses a rotating polygon mirror and laser beams for effecting the optical scanning on a photosensitive drum. The polygon mirror used for such an image forming apparatus requires high precision. In particular, the mirror surfaces thereof must be precisely finished, and the inclinations of the mirror surfaces must be precisely controlled. Any variation of the inclinations between the mirror surfaces produces a significant error on a printed surface.
Referring to FIG. 6, the manner in which an inclination of the mirror surface affects the displacement of light beam spots on a photosensitive drum 9 will be described. As is shown in FIG. 6, a conventional image forming apparatus comprises an optical system including a polygon mirror 32, a motor 33 and a lens 34. When the resolution of the imaging forming apparatus is 300 dpi, the pitch of dots is approximately 85 .mu.m (25.4/300). In this case, the allowable error of the radiated light beam spots on the photosensitive drum should be approximately 20 .mu.m. When the maximum allowable inclination angle of the mirror surfaces of the polygon mirror is .alpha., the error e of the light beam spots on the photosensitive drum 9 is Ltan2.alpha., where L is the optical path length. When L is 180 mm, in order to obtain the error e of 20 .mu.m or less as is mentioned above, the maximum allowable inclination angle .alpha. must be as small as 0.003.degree. (approximately 11 seconds).
In order to meet this strict requirement, conventional polygon mirrors have been manufactured of hard aluminum base material by an ultra-precision processing method.
In recent developments involving high-precision plastic molding, a high-fluidity resin whose thermal contraction at molding has been greatly reduced has been developed. Such a resin can copy surfaces of the mold precisely, to the level of mirror surfaces, for example. Such a plastic molding resin and technique have been applied in the present field to manufacture a highly precise plastic polygon mirror. However, in the process of molding the polygon mirror, as is shown in FIGS. 7 and 8, there arises a problem as is described below.
In FIGS. 7 and 8, the reference numerals 35, 37, 38 and 39 denote a mold base, right and left slide molds, and an upper mold, respectively. These mold components are assembled into a unit as is shown in FIG. 8. A resin is injected from a resin inlet 36. After the resin is cooled and hardened, the molded polygon mirror is taken out with a pin while the mold components are separated in the directions shown by the arrows in FIG. 7. At this time, the side faces of the polygon mirror, that is, the mirror surfaces thereof, are liable to be scratched with the edges of the mold shown by dotted circles in FIG. 9.
The objective of the present invention is to solve the above-described problem and to provide an image forming apparatus using a polygon mirror having mirror surfaces which are not damaged when it is molded.